Medical devices may be used to deliver fluids, medications and/or nutrients into a patient's body in a controlled manner. For example, infusion pumps are commonly used devices that dispense a programmable volume and flow rate of medical fluids into patients. Infusion pumps must be tested on a periodic basis to ensure that the devices are functioning safely and accurately. The safety and performance indicators tested may include flow rate, volume delivered, and occlusion pressure.
In normal operation, a delivery volume and flow rate of infusion pumps are set by the user or clinical technician. Typical volumes range from 1 mL to 1000 mL, at flow rates of 0.1 mL/hr to 1000 mL/hr. Typical infusion rates are generally in the 100 to 200 mL/hr range. Infusion pumps are required to have a safety feature that detects soft occlusions (restrictions which elevate infusion pressure) as well as hard occlusions (blockage) downstream in the administration set. When detected, in conventional infusion pumps, these events cause an audible alarm and/or other signal to be sent to the clinician or user. In the case of a hard occlusion, the infusion pump stops pumping fluids. This feature is very important because several unsafe outcomes can result if the infusion pump continues to pump when an occlusion exists, such as bursting of the administration set due to over-pressure, interruption of vital fluids or drugs to the patient, and dangerous sudden-flow conditions when the occlusion is cleared without alarm. Due to the compliance of the tubing in the administration set, extra volume known as a bolus can build up between the infusion pump and patient administration site upon an occlusion event. In some cases, this bolus volume can be potentially dangerous to the patient if the occlusion (blockage) is removed as it will suddenly be infused into the body. Certain drugs may be dangerous if the infusion rate is suddenly increased in such a manner, especially in pediatric or infant patients. Many pumps now include an auto-backup function that reverses the infusion pump to clear this bolus volume upon hard occlusion.
There are several conventional techniques and devices used for administering the safety and performance tests of infusion pumps. Some basic techniques for testing volume delivered include the use of calibrated graduated cylinders, burettes, or electronic balances. In the case of the electronic balance, the volume is indirectly measured by measuring the mass of the dispensed fluid and then calculating its volume based on the fluid's density. Average flow rates are measured via a stopwatch in tandem with one of the above volume measurement devices. The flow rate is measured by taking the volume over the time measured between flow start and conclusion. For measuring occlusion pressure, pressure gauges or meters are typically used.
While these devices may inherently be considered accurate, they can be affected by user error or judgment. As an example, in the case of the graduated cylinder or burette, readability of the fluid level against a scale graduation can be greatly impacted by the user and technique. Another example of potential difficulty is fluid evaporation in electronic balance method, which can be overcome, but presents additional complexity and sources of error. Additionally, these methods can be very time consuming and have inherent limitations, such as the inability to measure instantaneous flow.
Several devices have been designed over the years to incorporate and improve these manual methods into more automated electronic systems. These systems are commonly known as infusion pump analyzers. Some of these even combine testing functions, such as volume/flow measurement plus occlusion pressure measurement. These devices vary greatly in measurement techniques used in their designs. These techniques generally include electronic burette systems, injected bubble tracking systems and fluid metering pumps. Electronic burette systems use a series of photosensors installed along a precision glass tube to measure volume and flow. These systems use an electronically actuated valve to alternate the fluid path to the burette between intake and exhaust ports so that it can dump the previous volume sample and get ready for the next. A pressure sensor is located before the valve in the intake port to measure fluid pressure and for occlusion testing.
Injected bubble systems use a similar electronic burette design, except that these systems locate the glass tube horizontally and intentionally inject an air bubble into the incoming fluid stream to measure the volume and flow rate. These systems require several valves, in addition to the mechanicals required to create the air bubble.
Fluid metering pumps use a pump mechanism to meter or measure the volume. These pumps have a pressure sensor on the inlet side to sense the incoming pressure, which is also used for occlusion pressure measurement. During flow and volume testing, the pressure sensor reading is used to control the pump speed so that the net pressure is nulled. This system requires no valves as the valve is built in to the pump itself.
Each of these systems has related strengths and weaknesses. For example, electronic burette and injected bubble designs can be fouled by improper fluids such as salines or glucose solutions. These can build up a film on the inside of the glass walls, disrupting the ability of the photosensors to measure the fluid. Additionally, these systems are prone to measurement inaccuracy or disruption by air bubbles, which may require complete reset of the test being undertaken. Typically manufacturers specify specially prepared test solutions containing purified water (e.g., deionized water) and a surfactant (1% micro-90 cleaning solution) to ensure valid tests. The surfactant is recommended to be used to keep the glass walls clean. Both systems have to be well primed and clear of bubbles before testing can begin. Both systems are delicate due to the special glass assemblies which require great care in handling and transport. In addition, some test procedures and specifications require the ability for the infusion test to be done at varying pressure conditions (backpressure or vacuum) to test additional pump safety functions. These systems do not have the ability to generate this backpressure or vacuum due to their passive design, and thus require these pressures be generated externally by the technician.
Fluid metering pump designs are vastly different as these designs are not sensitive to bubbles, and can easily generate a backpressure or vacuum. However, these pump designs are usually very costly due to the specialized materials used for construction such as precision machined ceramics. The conventional design uses a piston that directly acts within a bore or liner. The piston is actuated with an offset drive mechanism that is adjusted or calibrated for volume pump cycle, which is generally fixed somewhere around 50 uL per motor revolution. This design is simple in that the design only requires one motor to achieve two mechanical motions that would typically require respective actuators. However, the design requires significant actuation power, and like the electronic burette system, occludes (blocks) the inlet port while evacuating or exhausting the last sample. This temporary partial occlusion is an occlusion also of the infusion device under test (DUT), which can actually impact infusion pump operation. However, this effect may lead to potentially inaccurate or unrealistic flow parameters (as compared to infusion into an actual patient) due to the temporary but reoccurring pressure interruptions. Another potential weakness in a typical fluid metering design relates to the fixed cycle volume. If graphed over angular position of the actuating motor, the pump volume or displacement would appear sinusoidal. Thus, to resolve sub-cycle volumes (under 50 uL for example above), the angular position of the motor has to be well regulated and the cycle volume versus angular relationship well parameterized and constant.
Thus, current technology infusion pump analyzers for testing infusion pumps may not adequately and/or accurately perform the specified tests, and can be sensitive to setup and operation.